In order to operate a projection exposure tool economically it is desirable to image the structures onto the wafer with the shortest possible exposure time so as to thus achieve the highest possible throughput of exposed substrates in the form of semiconductor wafers. Therefore, high radiation intensities are involved in order to achieve sufficient exposure of each individual substrate. In particular, when using radiation in the ultraviolet or extreme ultraviolet (EUV) wavelength range the effect of intensive radiation can lead to radiation-induced changes in the properties of the individual optical elements of the projection objective of the projection exposure tool. These changed properties bring about aberrations in the projection objective. The extent of these aberrations depends upon the radiation dosage.
With dielectric mirrors used in EUV lithography, due to absorbed radiation, variations in temperature occur on the mirror surface which lead to deformations of the mirror surface.
These deformations lead to image errors in the lithographic imaging, and can change continuously during the imaging process.